Electric spark discharge device



g- 23, 1955 D. M. MCCALLUM 2,716,198

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United States Patent Oflice 2,7 16,198 Patented Aug. 23, 1955 ELECTRIC SPARK DISCHARGE DEVICE Donald Murdo McCallum, Hollinwood, Lanes, England, assignor to Ferranti Limited, Hollinwood, England, a British company Application January 16, 1950, Serial No. 138,925

Claims priority, application Great Britain January 18, 1949 8 Claims. (31. 313-231 This invention relates to electric spark-discharge devices and specifically to a device in which a spark discharge between main electrodes at a voltage lower than that required to break down the gap between such electrodes is initiated by a pilot spark produced between a pilot or auxiliary electrode and a further electrode which may either be a second auxiliary electrode or one of the main electrodes. Such a spark-discharge device will hereinafter be referred to as a spark-discharge device of the type stated. The discharge between the main electrodes is conveniently of high current density, while the pilot spark is preferably produced by a relatively high voltage.

The action of the pilot spark in such device is to produce a certain amount of preliminary ionisation of the main spark gap, i. e. the gap in between the main electrodes, in order to render the path of the main spark more conductive and so allow that spark to occur. In known forms of spark-discharge devices of the type stated, however, the preliminary ionisation set up by the pilot spark is apt to be irregular with the result that the initiation of the main spark is uncertain, particularly in its timing instant with relation to the auxiliary or pilot spark, unless an uneconomically high voltage is used to produce the latter.

One object of the present invention is to provide a spark-discharge device of the type stated in which the initiation of the main spark discharge is effected with certainty and time precision and without the use of an .uneconomically high voltage for producing the pilot spark.

Another object of the invention is to provide a sparkdischarge device of the type stated comprising means for causing the projection into the gap between the main electrodes of at least some of the air or surrounding gas which is ionised by the pilot spark-discharge.

A further object of the invention is to provide a sparkdischarge device of the type stated comprising two main electrodes having electrode tips defining a main spark gap therebetween, an auxiliary electrode having an electrode tip adjacent to but in ofi-set relationship with the aforesaid main gap and a tube of insulating material surrounding the tip of said auxiliary electrode and extending beyond such tip towards the main spark gap with the bore of said tube aligned on said main spark gap.

In order that the above and other objects and advantages of the invention may be more thoroughly understood a number of embodiments will now be described with reference to the accompanying drawing in which:

Fig. l is a perspective View of one form of sparkdischarge device in accordance with the present invention,

Fig. 2 is a cross-sectional elevation taken in the coincidental planes of the main and auxiliary spark gaps,

Fig. 3 is a plan view of the device shown in Figs. 1 and 2.

Fig. 4 is a circuit diagram showing the manner of operating the device according to the invention, while Fig. 5 is a perspective view, somewhat similar to Fig. 1, showing a modification.

Referring first to Figs. 1, 2 and 3 the device there shown comprises a base-plate 11), shown in the form of a disc, of suitable insulating material. Carried by this disc at two diametrically opposed points are two conductive rods 11, 12 disposed with their length dimensions lying normal to the side surfaces of the base-plate '10 and projecting through the latter on each side. On one side of the base-plate each of these rods 11, 12 is bent at right angles towards the other to form opposed electrode tips 13, 14 whose respective end-faces face one another to define the main spark gap 15 therebetween. The pilot or auxiliary electrode consists of a third conductive rod 16 of straight cylindrical shape. This rod is :passed through the centre of the base-plate 10 so as to lie parallel with and in the same plane as the rods 11, 3.2 with one .of its end faces 17 disposed in alignment with but in ofi-set relationship to the main spark gap 15. This rod lies wholly outside the main spark gap. Surrounding the .rod 16 is a straight length of insulating tube 18 which is similarly held at the centre of the base-plate 10 in coaxial relationship with the electrode .rod 16. Conveniently, as shown in Fig. 2, the tube 18 is secured within a bore in the base-plate 10 and the electrode rod 16 itself held within the bore of the .tube 13 by .a further interposed insulating sleeve 19.

The tube 18 is .of a refractory material, preferably silica or quartz, and is fitted over the operating tip formed by the end face 17 of the pilot electrode 16 so as to surround such tip and extend beyond the latter towards the main spark gap 15. The common axis of the rod 16 and tube 18 is disposed so as to intersect the axis of the main gap 15, but the tube 18 is terminated so that its upper end 20 lies clear of and does not impede the main gap path 15. The inner diameter of the tube 18 is slightly larger than the outer diameter of the rod 16.

As shown in Fig. 4, one manner of operating this device is to arrange for the application of a relatively lowvoltage but high-current power supply source 22 across the main electrodes 11, 12, by way of a series resistor 23 and with a large capacity condenser 24 connected directly across such electrodes. One side of the supply,

i. e. one of the main electrodes 11, is conveniently.

grounded as shown. A high-voltage pilot or triggering source 25 is connected between the auxiliary or pilot elcctrode 16 and one of the main electrodes 11, 12 preferably the, grounded electrode 11.

Conveniently the low-voltage high-current power supply 22 is of the order of one kilovolt, while the high-voltage supply source 25 may be of the order of 10 kilovolts in the form of short-duration steep-sided pulses whose timing may be accurately related to some other occurrence within the apparatus with which the spark gap described is being used.

In the operation of the device the voltage across the condenser 24 supplied by way of the feed resistance 23 from the source 22 is insufficient to break down the main spark gap 15 so as to initiate a spark discharge thereacross in the absence of any preliminary ionisation. To produce a main spark discharge a high-voltage pulse from the source 25 is applied between the electrode 16 and the main electrode 11 to set up a pilot spark between these electrodes. Ions are produced along the path of this spark and in consequence in the column of air within the tube 18 between the upper end 20 thereof and the electrode tip face 17 of the electrode rod 16. This air column is heated by the discharge and accordingly expands rapidly and, being partially confined by the tube 18, issues explosively from the open end 20 of the tube 18 in a direction towards the main gap 15. Ionised air is thus projected into the main gap to provide the requisite conditions for allowing such gap to be broken down by the relatively low voltage set up between these main electrodes by the condenser 24 charged from thesource 22. 7

When the main discharge is initiated in this manner with the arrangements shown in Fig. 4, the main arc cannot bemaintained by the supply 22 owing to the large .voltage drop across the resistance 23. The main discharge therefore ceases when the voltage of the condenser 24 drops below a certain level and the arc is automatically extinguished. The condenser 24 then commences to recharge and is held in thiscondition until a further main spark discharge is initiated by the application of a further high-voltage triggering pulse to the auxiliary electrode 16. The arrangement so far described necessarily involves a' common connection between the source of the highvoltage triggering pulses and that of the low-voltage-main arc supply as one of the main electrodes is used as a counterelectrode for the auxiliary or pilot spark discharge. This may be avoided however by providing a second pilot electrode and Fig. shows one embodiment of this modification. In Fig.5 the insulating base-plate 10 carries main electrodes 11,12 as before, together with an auxiliary electrode 16 surrounded by an insulating tube 18 in the manner already described. In this example, however, a further auxiliary electrode rod 30 is also carried in the disc 10 at a suitably displaced position and extends parallel with the other electrode rods to a point beyond the main gap where it is bent at right angles to provide a limb 31 lying'parallel with the plane of the disc 10, the free end of this limb being again bent at right angles to form an electrode tip 32 which is in alignment with'the electrode tip face 17 of the first auxiliary electrode rod 16, but is upon the oppoiste side of the main gap 15 with respect thereto. The respective axes of the main and auxiliary spark gaps 15 and 33 intersect one another at right angles as indicated at 34. The particular embodiments described are constructed for use in air, e. g. in free air and particularly for use as a spark-producing device. With such arrangements a surrounding envelope is not normally required,but clearly the same principles may be applied to a spark discharge device suitable for a variety of other purposes, and which is enclosed within a surrounding envelope. rounding envelope isemployed it may obviously be provided with a gaseous filling other than air and may be at' some modified pressure other than that of the existing atmospheric pressure.

" 'It will of course be understood changes may bemade in the form, details, Proportions and arrangements of the various parts without departing from the scope of my invention as defined in the appended claims.

) I claim: 3

p 1. An electric spark discharge device of the type stated comprising two main electrodes each provided with electrode tips defining between them a main spark-gap, an

auxiliary pilot-spark electrode having an electrode tip gap so that its bore, if extended, would intersect said :mai n spark gap.

3...An electric spark-discharge device comprisingan insulating support body, twoelectrode rods carried by said Where a surlying adjacent to but offset from a point in said main of said main spark-gap and a tube of insulating material surrounding one of said'auxiliary electrodes and extending from the electrode tip thereof towards said main spark insulating support body, said rods each being formed at one end to define opposed electrode tips constituting a main spark-gap, a third electrode rod carried by said insulating support body and disposed with its longitudinal axis at right angles to but in a'common plane with the path of direction of said main spark-gap, said third electrode rod terminating at one end in an electrode tip lying near to but whollyoutside of said main sparkgap and an insulating tube surounding said third electrode rod and extending beyond said electrode tip thereof towards said main spark-gap and terminating in an open orifice end adjacent to said main spark-gap.

4. An electric spark-discharge device according to claim 3 wherein said insulating tube is of refractory material having a bore diameter justs'lightly greater than the diameter of said third electrode rod.

5. An electric spark-discharge device comprising an insulating support body, two electrode rods carried by said insulating support body, said rods each being formed at one end to define opposed electrode tips constituting a main spark-gap, a third electrode rod carried by said insulating support body and disposed with its longitudinal axis at right angles tobut in a common plane with the path direction of said main spark-gap, said third electrode rod terminating at one end in an electrode tip lying wholly outside said main spark-gap, a fourth electrode rod carried by said insulating support body and terminating at one end .in an electrode tip lying wholly outside said main spark-gap and in diametrically opposed relationship, relative to .the main spark gap, to the electrode tip of said third electrode rod, and an insulating tube surrounding said third electrode rod and extending beyond the electrode tip thereof towards saidmain spark-gap and terminating in an open orifice end adjacent to but wholly clear of said main spark-gap.

6. Anelectric spark-discharge device of the type stated comprising a support body of insulating material, two electrode rods carried by said body and disposed in parallel relationship, said rods being each bent at one of their ends towards each other to define opposed electrode tips of a main spark-gap lying at right angles to the length dimension of said rods, a third electrode rod carried by said insulating body in parallel relationship to said first rods and in between the latter, said third electrode .rod

terminating short of said main spark-gap in an electrode tip surface lying adjacent to but offset from said main spark-gap and in between said first electrodes and a tube of refractory insulating material surrounding said third electrode rod and extending nearer towards such main spark gap than-said electrode tip of said third electrode rod, said tube terminating in an open end lying wholly clear of said main spark-gap.

7. An electric spark discharge device of the type stated comprising a support body of insulating material, two electrode rods carried by said body and disposed inparallel relationship, said rods being each bent at one of their ends towards each other to define opposed electrode tips of a main spark-gap lying at right anglesto the length dimension of said rods, a third electrode rod carried by said insulating body in parallel relationship to said first rods and in between the latter, said third electrode rod terminating short of said main spark-gap in an electrode tip surface lying adjacent to but offset from said main spark-gap at a point midway between said opposed first electrode tips, a fourth electrode rod carried by said insulating body, said fourth electrode rod .being bent to provide a further electrode tip opposing the electrode tip of said third electrode rod on-the opposite side of the main spark gap and an insulating tube surrounding said third electrode rod and extending nearer towards said main spark gap than said electrode tip of said third electrode.

8. An electric spark discharge device comprising a pair of main spark gap electrodes defining a main spark gap therebetween and at least one auxiliary electrode for producing a pilot spark discharge, said auxiliary electrode being separate from and laterally displaced with respect to the discharge path of said main spark gap and which includes means associated with said pilot electrode for effecting partial confinement of the gaseous medium surrounding said auxiliary electrode and through which said pilot spark discharge takes place, said confining means having an outlet for relieving the pressure set up by the heating of said confined medium by the pilot spark dis charge and said outlet being directed to eject the heated and ionised gaseous medium along a path which crosses the discharge path of said main spark gap.

References Cited in the file of this patent UNITED STATES PATENTS Massie July 2, 1907 Rentschler Aug. 30, 1932 Inman Mar. 1, 1938 Adam Mar. 3, 1942 Haine May 14, 1946 Tonks July 30, 1946 Tonks et a1. July 30, 1946 Colson May 24, 1949 Latour et al May 23, 1950 

